TITLE: GCN CIRCULAR NUMBER: 7169 SUBJECT: Transient in NGC 2770: spectroscopic evidence for a SN DATE: 08/01/11 21:01:14 GMT FROM: Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center D. Malesani, J. Hjorth (DARK), P. Jakobsson (Univ. Hertfordshire), P.M. Vreeswijk, C.C. Thoene, J.P.U. Fynbo, D.J. Watson, J. Sollerman (DARK), N.R. Tanvir (Univ. Leicester), T. Stanke (ESO), report on behalf of a larger collaboration: We observed the optical transient in NGC 2770 (Berger & Soderberg, GCN 7159; Deng & Zhu, GCN 7160), denoted SN 2008D (Li & Filippenko, IAUC 1202), with the ESO VLT equipped with FORS2, starting on 2008 Jan 11, 07:17 UT. Two spectra, at low and moderate resolution, were acquired, covering the wavelength ranges ~3700-6350 and ~3800-9000 AA. The exposure times were 10 and 15 minutes, respectively. The two spectra, after flux calibration, agree very well in the overlap region, and the peak S/N is ~60 and ~40, respectively. We detect a strong, red continuum revealing several broad features and bumps. The afterglow spectrum does not display a power-law shape. We also detect Na I D in absorption at z = 0.0069. This redshift is very similar to that of the host z=0.0071, as measured for the Halpha and Hbeta lines in emission superimposed on the transient. The detection of Na I D confirms that the object is of extragalactic origin. It also implies that it is subject to significant extinction. Although uncertain, using the relation by Munari & Zwitter (1997, A&A, 318, 269), we estimate E(B-V) to be larger that ~0.5 mag based on the equivalent width of the 5890 A line EW = 0.67 AA. Such relatively large extinction is consistent with the detection of excess column density in the X-ray spectra of the flare (Berger & Soderberg, GCN 7159). The spectral shape of the transient rules out a significant nonthermal afterglow component. The presence of broad features is reminiscent of SN spectra, although it is not straightforward to identify the involved transitions. The lack of prominent Silicon and Hydrogen lines suggests the SN is not of type Ia or II. By exclusion, we suggest the transient is a SN of type Ib/c. The features are not as broad as in the earliest spectra of GRB-associated SNe such as SN1998bw and SN2006aj. The luminosity of the event is also comparable to that of other core-collapse SNe associated with GRBs. The object had V~19.3 approximately 1.5 days after the X-ray transient (Thoene et al., GCN 7161). Taking into account the large extinction (> 1.5 mag in V), the luminosity is comparable with that of core-collapse SNe soon after the explosion. As noted by several authors, the available light curve is also consistent with a SN (Valenti et al., GCN 7163; Kong et al., GCN 7164, Li & Filippenko, CBET 1202). In conclusion, the bright absolute magnitude of the object, M_V < -14, the broad spectral features, and the lack of H and Si strongly suggest that the transient in NGC 2770 is a SN of type Ib/c. Lastly, we note that the transient does not seem to lie in an intense star forming region of its host, as suggested by the modest nebular features detected in our spectrum and the absence of prominent emission at the position of the optical transient (D'Elia et al., GCN 7162) in the archival SDSS images. We caution that strong extinction may affect this arguments. We acknowledge excellent support of the observing staff at Paranal, in particular Cedric Ledoux, Linda Schmidtobreick, and Antonio de Ugarte Postigo. [GCN OPS NOTE(13jan08): Per author's request, the "IAUC 1202" citation was changed to "CBET 1202" (at the end of the 4th paragraph).]